Suppress ZMQ_EXTENDED_CONSTEXPR on gcc with partial C++14 support

Author: gin-ahirsch

zmq.hpp

@@ -94,7 +94,7 @@
 #define ZMQ_CONSTEXPR_VAR const
 #define ZMQ_CPP11_DEPRECATED(msg)
 #endif
-#if defined(ZMQ_CPP14) && (!defined(_MSC_VER) || _MSC_VER > 1900)
+#if defined(ZMQ_CPP14) && (!defined(_MSC_VER) || _MSC_VER > 1900) && (!defined(__GNUC__) || __GNUC__ > 5 || (__GNUC__ == 5 && __GNUC_MINOR__ > 3))
 #define ZMQ_EXTENDED_CONSTEXPR
 #endif
 #if defined(ZMQ_CPP17)